Serial printing systems are known in which a rotary print wheel is mounted on a carriage for translation across the width of the print area during printing. The carriage is typically translated from left to right through a series of cables and pulleys driven incrementally by a D.C. motor controlled by a suitable electronic control system. As the carriage is translated from print position to print position along a line, the print wheel is rotated so that the character pad bearing the next character to be printed is aligned with the striking end of a print hammer, also mounted on the carriage, when the carriage is momentarily stopped. When the carriage stops, the print hammer is actuated to impress the character borne by the pad against an inking ribbon and the face of the printing media to print that character. After the print hammer rebounds, the carriage is translated to the next print position, the print wheel is rotated so that the proper character pad is aligned with the print hammer, and the next character is printed. This process continues until a complete line has been so printed, after which the carriage is returned to the starting position in preparation for printing the next line of characters.
Proper operation of such printing systems depends on a number of factors. Firstly, accurate positioning mechanisms must be employed for both the carriage translation mechanism and the print wheel rotating mechanism to insure that the characters are printed at exactly the right location with the proper orientation.
Another factor affecting performance of such printing systems is the manner in which the print hammer is operated during the actual printing of a character.
U.S. Pat. No. 4,118,129 describes a serial printing system including print wheel and carriage servo motors to achieve extremely flexible and precise print wheel and carriage positioning at high speeds. The system also provides a variable print hammer striking force capable of being adjusted over a wide range of magnitudes. The print wheel and carriage servo motors are each provided with an opto-electronic position sensing circuit for generating precise position control signals for the associated electronic control system.
The electronic control system of said U.S. Pat. No. 4,118,129 includes a character register for storing data representative of a character to be printed supplied from an associated source; a memory device for storing both character selection information and also hammer intensity information associated with the individual characters on the print wheel; a print wheel position counter for generating signals indicative of the instantaneous angular position of the print wheel; an arithmetic unit for generating signals representative of the angular position of the desired character on the print wheel relative to the character printing position, and the initial optimum direction of rotation of the wheel; a carriage position counter for indicating the instantaneous position of the print wheel character relative to the next character print position along a line; and a timing and control unit responsive to the signals generated by the aforementioned units for generating control signals for a hammer drive unit, a print wheel drive unit and a carriage drive unit, as well as timing signals for clocking the print wheel and carriage position counters and a "Select" signal for specifying read out of the character selection or the hammer intensity information from the memory.
The print wheel drive unit and the carriage drive unit each comprise a servo control system operable in both a position, or detent, mode and a rate mode, the former being employed when the print wheel or the carriage is statically positioned for printing, the latter being employed during motion of the print wheel or the carriage.
In the rate mode, position reference signals from the associated position sensing circuit are processed to provide a plurality of sequentially sampled reference signals indicating instantaneous velocity of the movable member while the position signals from the position counter are converted into a correction signal representative of a predetermined ideal motor velocity. The instantaneous velocity signals and the ideal velocity signals are combined to provide an optimum error correction voltage.
The serial printer described in the aforementioned patent relied upon hard-wired control logic to implement the many control functions and operations required. An improved version of that printer wherein a microprocessor is utilized in place of portions of hard-wired control logic for powerful flexibility is the Sprint 5 daisywheel printer, manufactured by Qume Corporation, San Jose, Calif.
U.S. Pat. No. 4,315,198 discloses a digital servo system useful in serial printers for accurately positioning a movable member such as a rotary print wheel or carriage. Although the technique disclosed in this patent improves upon prior art linear servo control systems for controlling the movement of a print wheel and/or carriage by utilizing digital circuit techniques, the problem of minimizing the limitations imposed upon the overall system by the microprocessor bandwidth was not addressed. "Bandwidth" used in association with a microprocessor typically refers to the throughput capacity of the processor and depends both upon the microprocessor's processing speed (i.e., the number of instructions that may be executed by the microprocessor per unit of time) and also upon the amount of data that may be processed in parallel (i.e., the number of bits which a single microprocessor instruction can manipulate as a single word of data.)